1. Field of the Invention
The present invention relates to a vibratory flowmeter and method, and more particularly, to a vibratory flowmeter and method for determining an average flow rate.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flowmeters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness, and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flowmeter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries, emulsions, and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement, among other things. Vibratory flowmeters, including Coriolis mass flowmeters and densitometers, therefore employ one or more flow tubes that are vibrated in order to measure a fluid.
It is often desired to measure a flow rate of a flowing fluid. Where the fluid is flowing steadily, such flow rate measurement is straightforward. But where the flow is pulsating, such as in a pumped flow, then a flow rate measurement may reflect the periodic nature of the pulsating flow, with the flow rate measurement varying in amplitude, in time with the flow.
Many types of pumps will output a flow that is significantly periodic and consequently the flow will pulsate according to a period (or operational speed) of the pump. For example, the Texsteam 5000 series short stroke, chemical injection pump has a stroke rate as low as five strokes per minute with a ten percent “ON” duty cycle.
It is often desired that a flow rate measurement of a pulsating flow comprise an average flow rate measurement instead of an instantaneous flow rate measurement. It is often desired that the flow rate measurement comprise a substantially steady and representative flow rate measurement value. It is often desired that the flow rate measurement not vary periodically. A flowmeter user may not want, or be able to use, an instantaneous flow rate measurement.
One complicating factor in generating an average flow rate measurement for a pulsating flow is that a pulsating flow may have a periodic reverse flow. Another complicating factor is that users desire an average flow rate measurement that has a quick update rate and does not noticeably or appreciably lag the actual flow. Further, an average flow rate measurement may be complicated where the period between pulses is changing.
One prior art solution to measuring the flow rate of a pulsating flow has been to utilized flow damping. The flow damping comprises a software filtering (typically using an infinite impulse response (IIR) or finite impulse response (FIR) filter having a fixed number of filter coefficients). The filtering will smooth the instantaneous flow rate output, but is generic and non-adaptive to pulsating flow periods. The flow damping may operate to damp a periodic flow peak, wherein periodicity is somewhat reduced by flattening out and widening the flow peaks. The flow damping may operate to eliminate or reduce incidents of negative or reverse flow.
However, such damping includes drawbacks. One drawback is that such damping may create an average flow rate that has significant variation. A varying average flow rate may result because the flow damping is non-adaptive to the pulsating flow periods. Another drawback is that although the damping may decrease the average flow rate variation, doing so delays the average flow rate measurement by significantly more than the one period which can be achieved according to the vibratory flowmeter and method discussed herein.